Thomas Young, (born June 13, 1773, Milverton, Somerset, England—died May 10, 1829, London), English physician and physicist who established the principle of interference of light and thus resurrected the century-old wave theory of light. He was also an Egyptologist who helped decipher the Rosetta Stone.

In 1799 Young set up a medical practice in London. His primary interest was in sense perception, and, while still a medical student, he had discovered the way in which the lens of the eye changes shape to focus on objects at differing distances. He discovered the cause of astigmatism in 1801, the same year he turned to the study of light.

By allowing light to pass through two closely set pinholes onto a screen, Young found that the light beams spread apart and overlapped, and, in the area of overlap, bands of bright light alternated with bands of darkness. With this demonstration of the interference of light, Young definitely established the wave nature of light. He used his new wave theory of light to explain the colours of thin films (such as soap bubbles), and, relating colour to wavelength, he calculated the approximate wavelengths of the seven colours recognized by Newton. In 1817 he proposed that light waves were transverse (vibrating at right angles to the direction of travel), rather than longitudinal (vibrating in the direction of travel) as had long been assumed, and thus explained polarization, the alignment of light waves to vibrate in the same plane.

Young’s work was disparaged by most English scientists: any opposition to a theory of Newton’s was unthinkable. It was only with the work of the French physicists Augustin J. Fresnel and François Arago that Young’s wave theory finally achieved acceptance in Europe.

Young also studied the problem of colour perception and proposed that there is no need for a separate mechanism in the eye for every colour, it being sufficient to have three—one each for blue, green, and red. Developed later by the German physicist Hermann L.F. von Helmholtz, this theory is known as the Young–Helmholtz three-colour theory.

Having become interested in Egyptology, Young began studying the texts of the Rosetta Stone in 1814. After obtaining additional hieroglyphic writings from other sources, he succeeded in providing a nearly accurate translation within a few years and thus contributed heavily to deciphering the ancient Egyptian language.

Young also did work on measuring the size of molecules, surface tension in liquids, and on elasticity. He was the first to give the word energy its scientific significance, and Young’s modulus, a constant in the mathematical equation describing elasticity, was named in his honour.